Heating, Ventilation, and Air Conditioning (HVAC) is the technology of  indoor environmental quality.  The goal is to provide  thermal comfort and acceptable indoor air quality.  HVAC system design is a sub-discipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer.  Refrigeration is sometimes added to the field's abbreviation as HVAC&R or HVACR.  Ventilation (the V in HVAC) is the process of exchanging or replacing air in any space to provide high indoor air quality which involves temperature control, oxygen replenishment, and removal of moisture, odors, smoke, heat, dust, airborne bacteria, carbon dioxide, and other gases.  Ventilation removes unpleasant smells and excessive moisture, introduces outside air, keeps interior building air circulating, and prevents stagnation of the interior air.  Ventilation includes both the exchange of air to the outside as well as circulation of air within the building.  It is one of the most important factors for maintaining acceptable indoor air quality in High Performance Buildings.  A Complete definition of HVAC should be reviewed by contractors, as research shows approximately 50% of homeowners are not happy with their indoor environments.